This invention relates to the recovery of C.sub.3+ hydrocarbons from a feed stream generally containing, as essential components, hydrogen as well as C.sub.1 to at least C.sub.5 hydrocarbons. Conventionally, such a feed stream is cooled to provide a liquid fraction and a gaseous fraction. The liquid fraction is then fractionated by rectification into a product stream containing essentially C.sub.3+ hydrocarbons and a residual gas stream containing predominantly C.sub.2- hydrocarbons.
Such processes are used primarily for the separation of propane (optionally propylene and higher hydrocarbons as well) from refinery waste gases. The processing of refinery waste gases has become of interest lately from an economics viewpoint, since the market prices for LPG (C.sub.3 /C.sub.4) hydrocarbon mixture) have risen, while the demand for vacuum residues as well as heavy oil has decreased. For this reason, the poorly marketable heavy products are burned to cover internal fuel needs whereas readily marketable C.sub.3+ hydrocarbons are separated from the waste gas.
A process of the above-discussed type has been described (Oil and Gas Journal, May 10, 1982, pp. 127-313) wherein a refinery waste gas with the aforedescribed composition is first compressed to the required process pressure and, after separation of water, is cooled and partially condensed. The condensate is introduced into a rectification column wherein a liquid C.sub.3 hydrocarbon fraction and a gaseous C.sub.2- hydrocarbon fraction are obtained. The gas phase from the partial condensation step is expanded in a turbo-expander to result in a two phase fluid and said fluid is introduced into the rectification column as reflux.
Because the two phase fluid is employed as reflux, several disadvantages occur. On the one hand, the increase in concentration of lighter components in the rectification column requires the rectification column to be operated at relatively low temperatures and accordingly consumes a large amount of refrigeration energy. On the other hand, because the light proportion introduced into the separating column is a two-phase mixture, the expansion of the gas into the liquid-vapor range must be performed in an especially high-quality and expensive turbo-expansion device designed for this purpose.